1. Field of the Invention
The present invention relates to prism sheets, and particularly, to a prism sheet used in a backlight module.
2. Discussion of the Related Art
In a liquid crystal display device (LCD device), liquid crystal is a substance that does not illuminate light by itself. Instead, the liquid crystal relies on light received from a light source to display information. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
FIG. 7 depicts a typical direct type backlight module 100. The backlight module 100 includes a housing 11, a plurality of lamps 12 disposed above a base of the housing 11, a light diffusion plate 13 and a prism sheet 10 stacked on top of the housing 11 in that order. Inner walls of the housing 11 are configured for reflecting light upwards. The light diffusion plate 13 includes a plurality of dispersion particles (not shown) therein. The dispersion particles are configured for scattering light, thus enhancing the uniformity of light exiting the light diffusion plate 13.
Referring to FIG. 8, the prism sheet 10 includes a base layer 101 and a prism layer 103 formed on the base layer 101. The prism layer 103 contains a plurality of parallel prism lenses 105 having a triangular cross section. The prism lenses 105 are configured for collimating light to a certain extent. Typically, a method of manufacturing the prism sheet 10 includes the following steps: first, a melted ultraviolet (UV)-cured transparent resin is coated on the base layer 101, then the melted ultraviolet-cured transparent resin is solidified into the prism lenses 105.
In use, unscattered light from the lamps 12 enters the light diffusion plate 13 and becomes scattered. The scattered light leaves the light diffusion plate 13 and enters the prism sheet 10. The scattered light then travels through the prism sheet 10 before refracting out at the prism lenses 105 of the prism sheet 10. Thus, refracted light that leaves the prism sheet 10 is concentrated at the prism layer 103 and increases the brightness (illumination) of the prism sheet 10. The refracted light then propagates into an LCD panel (not shown) disposed above the prism sheet 10.
When the light is scattered in the light diffusion plate 13, scattered light enters the prism sheet at different angles of incidence. Referring to FIG. 9, when scattered light enters the prism sheet 10 at different angles of incidence, the scattered light generally travels through the prism sheet 10 along three light paths. In the first light path (such as a1, a2) the light enters the prism sheet at small angles of incidence and refracts out of the prism lenses with the refracted path closer to the normal to the surface of the base layer. In the second light path (such as a3, a4) the light enters the prism sheet 10 at angles of incidence larger than the first light path and refracts out of the prism lenses 105 with the refracted path being closer to the normal to the surface of the prism lenses 105. Both the first light path and the second light path contribute to the brightness of the LED and the light utilization efficiency of the backlight module 100. However, in the case of the third light path (such as a5, a6), the light enters the prism sheets at angles greater than the second light path, such that when the refracted light in the third light path leaves the prism sheet 10 at the prism lenses 105, the refracted light impinges on the surface of adjacent prism lens 105 and reenters the prism sheet 10. Thus, light traveling along the third light path will eventually reenter the prism sheet 10 and may exit the prism sheet 10 on the same side the light entered. This third light path does not contribute to the light utilization efficiency of the backlight module 100. Further, the third light path may interfere with or inhibit other incident light resulting in decreasing brightness of the backlight module 100.
What is needed, therefore, is a new prism sheet and a backlight module using the prism sheet that can overcome the above-mentioned shortcomings.